Field of the Invention
The present invention relates to an image forming system and a conveying control method.
Description of the Related Art
An image forming apparatus using electrophotographic process technology (printer, copier, facsimile, or the like) forms an electrostatic latent image by emitting a laser beam based on image data to a photo-conductor charged (exposure), in general. Then, the image forming apparatus visualizes the electrostatic latent image to form a toner image by supplying toner from a developing device to the photo-conductor (image carrier) on which the electrostatic latent image is formed. Further, the image forming apparatus transfers the toner image directly or indirectly to a sheet, and then forms an image on the sheet by heating and pressurizing the toner image with a fixing device for fixing the image. Then, the sheet passing through the fixing device passes through an ejection roller pair for ejection, and is ejected on a sheet ejection tray.
In addition, at time of continuous printing in which image forming is continuously performed to plural sheets, the sheets as plural output objects are stacked and placed on the sheet ejection tray. At this time, in the toner image (toner layer) formed on each sheet on the sheet ejection tray, charge given at time of transfer may remain in the toner layer even after fixing processing, for example. Since the toner layer has a high resistance value, in particular in an image forming apparatus having a fast image forming process speed, the sheet passing through the fixing device is ejected on the sheet ejection tray while sufficient self discharge is not performed. When the plural sheets are continuously stacked and placed on the sheet ejection tray, air between the sheets comes out due to weight of the sheets, and a distance between the toner layers is decreased. As a result, there has been a problem that electrostatic attraction force between the sheets is increased with a lapse of time, and a phenomenon (hereinafter referred to as “ejection sheet sticking phenomenon”) is caused in which adjacent sheets are stuck together via the toner image on the sheet. The ejection sheet sticking phenomenon is caused significantly in a case in which a temperature and humidity environment of the image forming apparatus is a low temperature and low humidity environment, or in a case in which a printing mode is a duplex printing mode.
To the above problem, a configuration has been devised in which a discharge device is provided at a downstream side of the fixing device in a sheet conveying direction, and the charge remaining in the toner layer on the sheet is removed. The discharge device includes a configuration including a discharge member (for example, a discharge roller pair capable of sandwiching a sheet), and a discharge voltage applying unit that applies a discharge voltage for discharging static electricity from the sheet to the discharge member.
In JP 2013-227088 A, a sheet loading device has been devised that prevents degradation of image quality due to contact between the sheets while ensuring appropriate loading of the sheet. The sheet loading device described in JP 2013-227088 A applies to a charging unit a correction voltage according to a potential of a sheet before loading onto a loading unit, and controls the charging unit to charge static electricity on the sheet.
In addition, an image forming system has been known in which the above image forming apparatus is connected to a post-processing device, and post-processing, such as center folding processing, binding (saddle stitching, side stitching) processing, is performed to the sheet on which the toner image is formed by the image forming apparatus, and a booklet is created. In the image forming system, to prevent occurrence of the ejection sheet sticking phenomenon after the post-processing, a configuration has been devised in which the discharge member is provided on a sheet conveying path between the image forming apparatus and the post-processing device in the sheet conveying direction, and the charge remaining in the toner layer on the sheet is removed.
In the above image forming system, to gain post-processing time in the post-processing device, a measure is performed in which a space is made between the sheets by accelerating a conveying speed of the sheet from a conveying speed in the image forming apparatus on the sheet conveying path between the image forming apparatus and the post-processing device in the sheet conveying direction. However, in a case in which the sheet is accelerated during discharge of static electricity from the sheet by the discharge member, a value of discharge current (current for canceling the charge on the sheet) flowing to the sheet is not an appropriate value due to application of the discharge voltage to the discharge member during the acceleration, and static electricity in the sheet cannot be sufficiently discharged from an area to which the discharge current not having the appropriate value flows. As a result, there is a problem that the ejection sheet sticking phenomenon occurs in the booklet created by performing the post-processing, and turning a page (sheet) of the booklet is difficult.
In JP 2013-227088 A, although a configuration is described in which the correction voltage according to the potential of the sheet before loading is applied to the charging unit, there is no description or suggestion that the sheet is accelerated during application of the correction voltage. For that reason, the above problem cannot be solved by applying the technology described in JP 2013-227088 A as it is.